Integrated nitride and silicon carbide-based devices

ABSTRACT

A monolithic electronic device includes a first nitride epitaxial structure including a plurality of nitride epitaxial layers. The plurality of nitride epitaxial layers include at least one common nitride epitaxial layer. A second nitride epitaxial structure is on the common nitride epitaxial layer of the first nitride epitaxial structure. A first plurality of electrical contacts is on the first epitaxial nitride structure and defines a first electronic device in the first nitride epitaxial structure. A second plurality of electrical contacts is on the first epitaxial nitride structure and defines a second electronic device in the second nitride epitaxial structure. A monolithic electronic device includes a bulk semi-insulating silicon carbide substrate having implanted source and drain regions and an implanted channel region between the source and drain regions, and a nitride epitaxial structure on the surface of the silicon carbide substrate. Corresponding methods are also disclosed.

CLAIM OF PRIORITY

This application is a divisional of U.S. patent application Ser. No.11/410,768 filed on Apr. 25, 2006, now U.S. Pat. No. 7,875,910, whichclaims priority under 35 U.S.C. §120 as a continuation-in-partapplication of U.S. patent application Ser. No. 10/378,331, filed onMar. 3, 2003, now U.S. Pat. No. 7,112,860, the disclosures of which arehereby incorporated by reference herein as if set forth in theirentireties.

FIELD OF THE INVENTION

The present invention relates to nitride-based devices. In particular,the present invention relates to the monolithic integration of differenttypes of nitride-based devices on a common substrate, and resultingdevices.

BACKGROUND

Wide bandgap semiconductor materials, including Group III-nitrides, suchas gallium nitride, aluminum gallium nitride, indium nitride and alloysthereof, and silicon carbide, are desirable materials for thefabrication of high power, high temperature and/or high frequencydevices. These wide bandgap materials have high electric field breakdownstrengths and high electron saturation velocities as compared to othersemiconductor materials such as gallium arsenide and silicon.

Electrical circuits requiring high power handling capability (>20 watts)while operating at high frequencies such as radio frequencies, includingfor example, S-band (2-4 GHz) and X-band (8-12 GHz), have in recentyears become more prevalent. Because of the increase in high power, highfrequency circuits there has been a corresponding increase in demand fortransistors that are capable of reliably operating at radio frequenciesand above while still being capable of handling higher power loads.Previously, bipolar transistors and power metal-oxide semiconductorfield effect transistors (MOSFETs) have been used for high powerapplications but the power handling capability of such devices may belimited at higher operating frequencies. Junction field-effecttransistors (JFETs) were commonly used for high frequency applicationsbut the power handling capability of previously known JFETs may also belimited.

Recently, metal-semiconductor field effect transistors (MESFETs) havebeen developed for high frequency applications. The MESFET constructionmay be preferable for high frequency applications because only majoritycarriers carry current. The MESFET design may be preferred over currentMOSFET designs because the reduced gate capacitance permits fasterswitching times of the gate input. Therefore, although all field-effecttransistors utilize only majority carriers to carry current, theSchottky gate structure of the MESFET may make the MESFET more desirablefor high frequency applications.

In addition to the type of structure, and perhaps more fundamentally,the characteristics of the semiconductor material from which atransistor is formed also affects the operating parameters. Of thecharacteristics that affect a transistor's operating parameters, theelectron mobility, saturated electron drift velocity, electric breakdownfield and thermal conductivity may have the greatest effect on atransistor's high frequency and high power characteristics.

Electron mobility is the measure of the ease of electron motion within asemiconductor media and is defined as the rate of change of electrondrift velocity with respect to electric field at a given electric field.In the past, semiconductor materials which have a high electron mobilitywere preferred because more current could be developed with a lesserfield, resulting in faster response times when a field is applied.Saturated electron drift velocity is the maximum velocity that anelectron can obtain in the semiconductor material. Materials with highersaturated electron drift velocities are preferred for high frequencyapplications because the higher velocity may translate to shortertransition times from source to drain.

Electric breakdown field is the field strength at which breakdown of theSchottky junction and the current through the gate of the devicesuddenly increases. A high electric breakdown field material may bepreferred for high power, high frequency transistors because largerelectric fields generally can be supported by a given dimension ofmaterial. Larger electric fields may allow for faster transients as theelectrons can be accelerated more quickly by larger electric fields thanby smaller.

Thermal conductivity is the ability of the semiconductor material todissipate heat. In typical operations, all transistors generate heat. Inturn, high power and high frequency transistors usually generate largeramounts of heat than low power transistors. As the temperature of thesemiconductor material increases, the junction leakage currentsgenerally increase and the current through the field effect transistorgenerally decreases due to a decrease in carrier mobility with anincrease in temperature. Therefore, if the heat is dissipated from thesemiconductor, the material may remain at a lower temperature and may becapable of carrying larger currents with lower leakage currents.Reliability physics also predicts a longer lifetime for a device thatoperates at lower operating temperature.

In the past, high frequency MESFETs have been manufactured of n-typeIII-V compounds, such as gallium arsenide (GaAs) because of their highelectron mobilities. Although these devices provided increased operatingfrequencies and moderately increased power handling capability, therelatively low breakdown voltage and the lower thermal conductivity ofthese materials have limited their usefulness in high powerapplications.

Silicon carbide (SiC) has been known for many years to have excellentphysical and electronic properties which should theoretically allowproduction of electronic devices that can operate at highertemperatures, higher power and higher frequency than devices producedfrom silicon (Si) or GaAs. The high electric breakdown field of about4×10⁶ V/cm, high saturated electron drift velocity of about 2.0×10⁷cm/sec and high thermal conductivity of about 4.9 W/cm-K indicate thatSiC would be suitable for high frequency, high power applications.

SiC-based MESFET structures and their fabrication are described in U.S.Pat. No. 5,270,554 to Palmour et al. and U.S. Pat. No. 5,925,895 toSriram et al., both of which are incorporated herein by reference as iffully set forth herein. SiC MESFET structures and fabrication are alsodescribed in U.S. application Ser. No. 09/567,717 filed May 10, 2000 byAllen, et al., the disclosure of which is incorporated herein byreference as if fully set forth herein.

In the III-nitride material system, a device of particular interest forhigh power and/or high frequency applications is the high electronmobility transistor (HEMT), which is also known as a heterostructurefield effect transistor (HFET). These devices may offer operationaladvantages under a number of circumstances because a two-dimensionalelectron gas (2DEG) is formed at the heterojunction of two semiconductormaterials with different bandgap energies, and where the smaller bandgapmaterial has a higher electron affinity. The 2DEG is an accumulationlayer in the undoped, smaller bandgap material and can contain a veryhigh sheet electron concentration in excess of, for example, 10¹³carriers/cm². Additionally, electrons that originate in thewider-bandgap semiconductor transfer to the 2DEG, allowing a highelectron mobility due to reduced ionized impurity scattering.

This combination of high carrier concentration and high carrier mobilitycan give the HEMT a very large transconductance and may provide a strongperformance advantage over metal-semiconductor field effect transistors(MESFETs) for high-frequency applications.

High electron mobility transistors fabricated in the galliumnitride/aluminum gallium nitride (GaN/AlGaN) material system have thepotential to generate large amounts of RF power because of thecombination of material characteristics that includes the aforementionedhigh breakdown fields, their wide bandgaps, large conduction bandoffset, and/or high saturated electron drift velocity.

In electronic communication systems, it is usually desirable to amplifysignals before transmission (power amplifier) or after reception (lownoise amplifier). It is also often desirable to filter such signalsimmediately before or after amplification. Directing the RF signal tospecific portions of a multifunction chip can be accomplished with amonolithic RF switch that is low-loss and can provide high isolation.Other types of non-power-amplifier circuits that would benefit frommonolithic integration with the power amplifier circuits are limitercircuits and phase shifters. In high frequency communication systems,such amplification may be performed efficiently using an amplifiercircuit incorporating a SiC MESFET or a Group III-nitride basedtransistor. Filtering may be efficiently performed using a SAW filter.

In order to minimize the number of circuit elements required toimplement a communication system and simplify its design, it isdesirable to integrate as many components as possible on a single chip.Attempts to integrate SAW devices with other devices have been made.However, such devices have typically required that the piezoelectriccrystal be bonded onto a semiconductor substrate (such as silicon) onwhich active electronic components are formed.

Acoustic wave devices form a class of electronic devices that processsignals that exist as acoustic (i.e. sound or compression) wavestraveling in piezoelectric crystals. Piezoelectric crystals arecharacterized by the fact that when the material is mechanicallystressed (i.e. compressed or placed under tension), an associatedelectric field is induced. Likewise, when an electric field is appliedto a piezoelectric crystal, the material becomes mechanically stressedin a predetermined manner. It is possible to exploit thesecharacteristics to perform many different functions with a piezoelectriccrystal.

For example, piezoelectric microphones convert acoustic waves travelingthough air into electronic signals. Piezoelectric speakers and buzzersperform the opposite function. Piezoelectric sensors detect changes inpressure, temperature, torque, humidity and/or a wide range of otherphenomena.

Common piezoelectric materials include quartz (SiO₂), zinc oxide (ZnO),barium titanate (BaTiO₃), lithium tantalate (LiTaO₃) and lithium niobate(LiNbO₃). However, other materials, most notably silicon carbide (SiC)and the Group III-nitride materials such as aluminum nitride (AlN) andGallium Nitride (GaN) are piezoelectric and may be used to form acousticwave devices.

When a time-varying electric field is applied to a portion of apiezoelectric crystal, the applied electric field induces an acousticwave that propagates through the crystal. Acoustic waves may travelthrough a piezoelectric material in a number of modes. For example,acoustic waves may travel through the body of the material—so-called“bulk” waves—or on the surface of the material. Waves that travel alongthe surface of the piezoelectric material are generally referred to assurface acoustic waves (or SAWs), and devices that process surfaceacoustic waves are referred to as surface acoustic wave devices, or SAWdevices.

A simple surface acoustic wave device comprises a piezoelectric crystalor a thin film of piezoelectric material on a substrate. Interdigitatedmetal stripes on the surface of the crystal form transmitting andreceiving electrodes. The metal electrodes convert electrical energyinto mechanical stress in the crystal and vice versa. Hence, theinterdigital electrodes formed on a piezoelectric material are referredto as interdigital transducers, or IDTs.

A simple surface acoustic wave device is illustrated in perspective inFIG. 10. The SAW device comprises a piezoelectric film 2 formed on asubstrate 1. A metal (usually aluminum) is deposited on the film andpatterned using standard photolithographic or liftoff techniques to formthe input IDT 3 and the output IDT 4. The thickness of the piezoelectricfilm is generally on the order of one SAW wavelength.

In operation, an electric signal may be applied to the input IDT 3. Theinput signal causes a surface acoustic wave to be induced in thepiezoelectric film 2 and propagate along the surface of the film 2towards the output IDT 4. The shape of the generated wave depends on theelectric signal applied to the input IDT, the design and orientation ofthe IDT fingers, and the piezoelectric material used. When the wavereaches the output IDT 4, a voltage is induced across the fingers of theIDT 4 which is then output from the device. The shape of the output waveis affected by the design of the output IDT 4.

FIG. 11 illustrates some design parameters for IDTs. The finger period Ddetermines the wavelength λ, of the SAW generated by the IDT. Thelinewidth L and space S of the fingers are generally equal to λ/4. Thenumber of fingers determines the coupling efficiency of the IDT, and thewidth W of the overlap of fingers affects the frequency response of thefinger pair. By changing the overlap of finger pairs within an IDT,various filter functions can be realized.

Surface acoustic wave devices have many different applications indigital and analog electronics. For example, surface acoustic wavedevices may be used as bandpass or bandstop filters, duplexers, delaylines, resonators and/or impedance elements among other things. They mayalso be used to perform digital functions such as convolution,correlation, pulse compression and/or digital filtering (for example inspread-spectrum communication systems) depending on the design of thedevice, and in particular depending on the layout of the interdigitaltransducers. The design and fabrication of surface acoustic wave devicesare described in Chapter 66 of K. Ng, Complete Guide to SemiconductorDevices, McGraw Hill (1995).

The velocity of surface acoustic waves in a device depends on thematerial, from which the device is constructed and the mode ofpropagation of the SAW. For example, the propagation velocity (alsocalled the SAW velocity) of first order Rayleigh-mode acoustic waves inGaN is around 3600 m/s, while the corresponding SAW velocity in AlN isabout 5800 m/s, and over 6800 m/s in SiC. For RF devices, the SAWvelocity determines the bandwidth of signals that can be processed bythe device. The fundamental operating frequency (f₀) of a SAW device isgiven by the formula:

$f_{0} = \frac{v}{\lambda}$

where v is the SAW velocity and is the wavelength. As discussed above,the wavelength of the device is determined by the finger period of theIDT. The width and spacing of IDT fingers (and thus the finger period)is limited by the resolution of photolithographic techniques. Thus, fora given finger period, increasing the SAW velocity increases thefundamental operating frequency of the device. Stated differently,having a higher SAW velocity permits a device to processhigher-frequency signals for a given device geometry. Accordingly, theGroup III-nitrides and SiC may be desirable piezoelectric materials forthe fabrication of SAW devices.

In addition to integrating SAW devices with nitride devices, it may bedesirable to integrate other types of nitride devices together on asingle substrate for more efficient manufacturing and/or operation.However, in the past it has proved difficult to provide multiple devicetypes on a common substrate due to the different epitaxial semiconductorstructures required for different types of devices.

SUMMARY OF THE INVENTION

Some embodiments of the invention provide a monolithic electronic deviceincluding a common nitride epitaxial layer, a first type of nitridedevice including a first epitaxial nitride structure on the commonnitride epitaxial layer, and a second type of nitride device, differentfrom the first type of nitride device, including a second epitaxialnitride structure on the common nitride epitaxial layer. A firstplurality of electrical contacts is on the first epitaxial nitridestructure and defines a first electronic device of the first type ofnitride device, and a second plurality of electrical contacts is on thesecond epitaxial nitride structure and defines a second electronicdevice of the second type of electronic device.

The first epitaxial nitride structure may include a nitride channellayer, a nitride barrier layer on the nitride channel layer, the nitridebarrier layer having a higher bandgap than the nitride channel layer.The nitride barrier layer and the nitride channel layer are configuredto cooperatively induce a two-dimensional electron gas at an interfacebetween the nitride channel layer and the nitride barrier layer.

The monolithic electronic device may further include a high bandgaplayer on the barrier layer and a silicon nitride layer on the highbandgap layer.

The second nitride epitaxial structure may include a layer ofAl_(x)Ga_(1-x)N (0≦x≦1) having a thickness of about 300 Å to about 1000Å.

The first electronic device may include a high electron mobilitytransistor. The second electronic device may include a surface acousticwave device, a diode, and/or a field effect transistor. In particular,the second electronic device may include source, drain and gatecontacts, and the gate and drain contacts of the second electronicdevice may be electrically coupled to form an anode.

The second epitaxial nitride structure may include a first layer ofn-type Al_(x)Ga_(1-x)N (0≦x≦1) on the first epitaxial nitride structureand a second layer of n-type Al_(x)Ga_(1-x)N (0≦x≦1) on the first layerof n-type Al_(x)Ga_(1-x)N (0≦x≦1). The first layer of n-typeAl_(x)Ga_(1-x)N (0≦x≦1) may have a surface charge density of about1×10¹⁴ cm², and the second layer of n-type Al_(x)Ga_(1-x)N (0≦x≦1) mayhave a doping concentration of less than about 1×10¹⁶ cm⁻³.

The common epitaxial layer may include a layer of semi-insulatingAl_(x)Ga_(1-x)N (0≦x≦1).

A monolithic electronic device according to further embodiments of theinvention includes a nitride epitaxial structure including a nitridechannel layer, and a nitride barrier layer on the nitride channel layer.The nitride barrier layer has a higher bandgap than the nitride channellayer, and the nitride barrier layer and the nitride channel areconfigured to cooperatively induce a two-dimensional electron gas at aninterface between the nitride channel layer and the nitride barrierlayer. An Al_(x)Ga_(1-x)N (0≦x≦1) layer is on the barrier layer, and afirst gate electrode is recessed through the Al_(x)Ga_(1-x)N (0≦x≦1)layer, the first gate electrode defining a first transistor device. Asecond gate electrode, defining a second transistor device, is on theAl_(x)Ga_(1-x)N (0≦x≦1) layer.

The monolithic electronic device may further include a silicon nitridelayer on the Al_(x)Ga_(1-x)N (0≦x≦1) layer. The first gate electrode andthe second gate electrode may both be recessed through the siliconnitride layer. The silicon nitride layer may include a high puritysilicon nitride layer.

The monolithic electronic device may further include a source/drainelectrode between the first gate electrode and the second gateelectrode. The source/drain electrode is recessed through theAl_(x)Ga_(1-x)N (0≦x≦1) layer and provides a source/drain contact forboth the first electronic device and the second electronic device.

A monolithic electronic device according to still further embodiments ofthe invention includes a bulk semi-insulating silicon carbide substrate,source and drain regions in a surface of the silicon carbide substrate,a channel region in the silicon carbide substrate between the source anddrain regions, and a nitride epitaxial structure on the surface of thesilicon carbide substrate. The source and drain regions and the channelregions may include implanted regions.

The monolithic electronic device may further include a first passivationlayer on the implanted channel region and the implanted source and drainregions of the silicon carbide substrate, and a first gate contact onthe silicon carbide substrate between the implanted source and drainregions, and a second gate contact on the nitride epitaxial layer. Thefirst gate contact extends through the passivation layer.

The first passivation layer may extend onto the nitride epitaxial layer,and the second gate contact may extend through the first passivationlayer.

The monolithic electronic device may further include a secondpassivation layer on the nitride epitaxial structure, the secondpassivation layer including a material different from the firstpassivation layer. The second gate contact may extend through the secondpassivation layer.

The first gate contact may be a control contact for a siliconcarbide-based transistor device and the second gate contact may be acontrol contact for a nitride-based transistor device, and theelectronic device may further include an interconnect metallizationconnecting the silicon carbide-based transistor device and thenitride-based transistor device.

Methods of forming a monolithic electronic device according to someembodiments of the invention include forming a first nitride epitaxialstructure including a plurality of nitride epitaxial layers including atleast one common nitride epitaxial layer, forming a second nitrideepitaxial structure on the common nitride epitaxial layer of the firstnitride epitaxial structure, forming a first plurality of electricalcontacts on the first epitaxial nitride structure, the first pluralityof contacts defining a first electronic device in the first nitrideepitaxial structure, and forming a second plurality of electricalcontacts on the first epitaxial nitride structure, the second pluralityof contacts defining a second electronic device in the second nitrideepitaxial structure.

Forming the first nitride epitaxial structure may include forming anitride channel layer, forming a nitride barrier layer on the nitridechannel layer, the nitride barrier layer having a higher bandgap thanthe nitride channel layer. The nitride barrier layer and the nitridechannel are configured to cooperatively induce a two-dimensionalelectron gas at an interface between the nitride channel layer and thenitride barrier layer.

The methods may further include forming a high bandgap layer on thebarrier layer, and forming a nitride layer, such as silicon nitrideand/or gallium nitride, on the high bandgap layer.

Forming the second epitaxial structure may include forming a first layerof n-type Al_(x)Ga_(1-x)N (0≦x≦1) on the first epitaxial structure, andforming a second layer of n-type Al_(x)Ga_(1-x)N (0≦x≦1) on the firstlayer of n-type Al_(x)Ga_(1-x)N (0≦x≦1). The first layer of n-typeAl_(x)Ga_(1-x)N (0≦x≦1) may have a surface charge density of about1×10¹⁴ cm⁻², and the second layer of n-type Al_(x)Ga_(1-x)N (0≦x≦1) mayhave a doping concentration of less than about 1×10¹⁶ cm⁻³.

Methods of fabricating a monolithic electronic device according tofurther embodiments of the invention include implanting source and drainregions in a surface of a bulk semi-insulating silicon carbidesubstrate, implanting a channel region in the silicon carbide substratebetween the implanted source and drain regions, and forming a nitrideepitaxial structure on the surface of the silicon carbide substrate.

The methods may further include forming a first passivation layer on theimplanted channel region and the implanted source and drain regions ofthe silicon carbide substrate, and forming a first gate contact on thesilicon carbide substrate between the implanted source and drainregions, and forming a second gate contact on the nitride epitaxiallayer. The first gate contact extends through the passivation layer.

The first passivation layer may extend onto the nitride epitaxial layer,and the second gate contact may extend through the first passivationlayer.

The methods may further include forming a second passivation layer onthe nitride epitaxial structure, the second passivation layer includinga material different from the first passivation layer. The second gatecontact may extend through the second passivation layer.

The first gate contact may be a control contact for a siliconcarbide-based transistor device and the second gate contact may be acontrol contact for a nitride-based transistor device, and the methodsmay further include forming an interconnect metallization to connect thesilicon carbide-based transistor device and the nitride-based transistordevice.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are schematic drawings illustrating embodiments of thepresent invention along with device precursors that may be anintermediate step in the fabrication of a device as illustrated.

FIGS. 2A-2B are schematic drawings illustrating embodiments of thepresent invention along with a device precursor that may be anintermediate step in the fabrication of a device as illustrated.

FIGS. 3A-3B are schematic drawings illustrating embodiments of thepresent invention along with a device precursor that may be anintermediate step in the fabrication of a device as illustrated.

FIG. 4 is a schematic drawing illustrating embodiments of the presentinvention.

FIG. 5 is a schematic drawing illustrating embodiments of the presentinvention.

FIGS. 6A-6C are schematic drawings illustrating embodiments of thepresent invention along with a device precursor that may be anintermediate step in the fabrication of a device as illustrated.

FIG. 7 is a schematic drawing illustrating embodiments of the presentinvention.

FIG. 8 is a schematic drawing illustrating embodiments of the presentinvention.

FIG. 9 is a schematic drawing illustrating embodiments of the presentinvention.

FIG. 10 is a perspective drawing illustrating a conventional surfaceacoustic wave device.

FIG. 11 is a schematic drawing illustrating the layout of aninterdigital transducer of a conventional surface acoustic wave device.

FIGS. 12A-12B are schematic illustrations of two device epitaxialstructures.

FIG. 13A is a schematic drawing illustrating embodiments of the presentinvention.

FIG. 13B is an equivalent circuit diagram of two devices formed on acommon substrate according to some embodiments of the invention.

FIGS. 14A-14B are schematic drawings illustrating embodiments of thepresent invention.

FIG. 15 is a schematic drawing illustrating embodiments of the presentinvention.

FIG. 16 is a schematic drawing illustrating embodiments of the presentinvention.

FIGS. 17A-17B are schematic drawings illustrating embodiments of thepresent invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which various embodiments ofthe invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout. Furthermore, the variouslayers and regions illustrated in the figures are illustratedschematically. Accordingly, the present invention is not limited to therelative size and spacing illustrated in the accompanying figures. Aswill also be appreciated by those of skill in the art, references hereinto a layer formed “on” a substrate or other layer may refer to the layerformed directly on the substrate or other layer or on an interveninglayer or layers formed on the substrate or other layer. Moreover, itwill be understood that when a first element or layer is described as“in electrical contact” with a second element or layer, the first andsecond elements or layers need not be in direct physical contact withone another, but may be connected by intervening conductive elements orlayers which permit current flow between the first and second elementsor layers.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”“comprising,” “includes” and/or “including” when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

Embodiments of the present invention are schematically illustrated as anintegrated SAW device/transistor 10 in the cross-sectional view of FIG.1A. In the embodiment illustrated in FIG. 1A, a transistor structure 10Aof a device 10 comprises a high electron mobility transistor (HEMT);however, it will be appreciated by those skilled in the art that thetransistor structure 10A (as well as the transistor structuresillustrated in connection with other embodiments of the inventiondescribed below) may comprise one or more other types of devices insteadof or in addition to a HEMT. For example, the transistor structure 10Amay comprise a MESFET, MOSFET, JFET or other device.

The integrated device 10 includes a substrate 12 that may, for example,be silicon carbide (SiC), such as semi-insulating silicon carbide of the4H polytype. Other silicon carbide candidate polytypes include the 3C,6H, and 15R polytypes. The term “semi-insulating” is used descriptivelyrather than in an absolute sense. In particular embodiments of thepresent invention, the silicon carbide bulk crystal has a resistivityequal to or higher than about 1×10⁵ Ω-cm at room temperature.

An aluminum nitride buffer layer 14 on the substrate 12 provides anappropriate crystal structure transition between the silicon carbidesubstrate and the remainder of the device. In the embodimentsillustrated in FIG. 1A, the aluminum nitride buffer layer 14 also formsa piezoelectric film on which a SAW device 10B is fabricated asdescribed in more detail below. The buffer layer 14 and subsequentGaN-based layers may be formed by MOCVD, MBE, or any other suitablegrowth technique for forming high-quality epitaxial layers.

Silicon carbide has a much closer crystal lattice match to Group IIInitrides than does sapphire (Al₂O₃), which is a very common substratematerial for Group III nitride devices. The closer lattice match mayresult in Group III nitride films of higher quality than those generallyavailable on sapphire. Silicon carbide also has a very high thermalconductivity so that the total output power of Group III nitride deviceson silicon carbide is, typically, not as limited by thermal dissipationof the substrate as in the case of the same devices formed on sapphire.Also, the availability of semi-insulating silicon carbide substrates mayprovide for device isolation and reduced parasitic capacitance.

Although silicon carbide is the preferred substrate material,embodiments of the present invention may utilize any suitable substrate,such as sapphire, aluminum nitride, aluminum gallium nitride, galliumnitride, silicon, GaAs, LGO, ZnO, LAO, InP and the like. In someembodiments, an appropriate buffer layer also may be formed.

As used herein, the term “Group III nitride” refers to thosesemiconducting compounds formed between nitrogen and the elements inGroup III of the periodic table, usually aluminum (Al), gallium (Ga),and/or indium (In). The term also refers to ternary and quaternarycompounds such as AlGaN and AlInGaN. As is well understood by those inthis art, the Group III elements can combine with nitrogen to formbinary (e.g., GaN), ternary (e.g., AlGaN, AlInN), and quaternary (e.g.,AlInGaN) compounds. These compounds all have empirical formulas in whichone mole of nitrogen is combined with a total of one mole of the GroupIII elements. Accordingly, formulas such as Al_(x)Ga_(1-x)N where 0≦x≦1or In_(y)Al_(x)Ga_(1-x-y)N where 0≦x≦1, 0≦y≦1 and x+y≦1 are often usedto describe them.

Appropriate semi-insulating SiC substrates are manufactured by, forexample, Cree, Inc., of Durham, N.C., the assignee of the presentinvention, and the methods for producing are described, for example,U.S. Pat. Nos. Re. 34,861; 4,946,547; 5,200,022; 6,218,680; 6,403,982;and 6,396,080, the contents of which are incorporated herein byreference in their entirety. Similarly, techniques for epitaxial growthof Group III nitrides have been described in, for example, U.S. Pat.Nos. 5,210,051; 5,393,993; 5,523,589; and 5,292,501, the contents ofwhich are also incorporated herein by reference.

Suitable structures for GaN-based HEMTs are described, for example, incommonly assigned U.S. Pat. No. 6,316,793 and U.S. application Ser. No.09/904,333 filed Jul. 12, 2001 for “ALUMINUM GALLIUM NITRIDE/GALLIUMNITRIDE HIGH ELECTRON MOBILITY TRANSISTORS HAVING A GATE CONTACT ON AGALLIUM NITRIDE BASED CAP SEGMENT AND METHODS OF FABRICATING SAME,” U.S.provisional application Ser. No. 60/290,195 filed May 11, 2001 for“GROUP III NITRIDE BASED HIGH ELECTRON MOBILITY TRANSISTOR (HEMT) WITHBARRIER/SPACER LAYER” and U.S. patent application Ser. No. 10/102,272,to Smorchkova et al., entitled “GROUP-III NITRIDE BASED HIGH ELECTRONMOBILITY TRANSISTOR (HEMT) WITH BARRIER/SPACER LAYER” the disclosures ofwhich are hereby incorporated by reference.

Continuing with the description of FIG. 1A, a channel layer 16, which insome embodiments is GaN, is formed on the buffer layer 14. The channellayer 16 is preferably undoped, but may be doped with various substancesin order to modify the electron concentration in the sheet charge regionor the behavior of the conduction band E_(c) and valence band E_(v) inthe area below the sheet charge region. In some embodiments, the channellayer 16 is about 1 to 5 μm thick. In some embodiments, the channellayer 16 may comprise a thick (1 to 5 μm) GaN-based buffer layer with athin (approximately 100 Å) high-quality channel region formed thereon.

The barrier layer 18 is formed on the channel layer 16. In someembodiments, the barrier layer 18 may comprise AlGaN, thereby forming aheterojunction between the channel layer 16 and the barrier layer 18.The barrier layer 18 preferably has an aluminum composition of between20% and 40% and may be doped with silicon at a doping concentration ofabout 2×10¹⁸ cm³. The barrier layer 18 may be between about 15 nm and 40nm in thickness, and is preferably about 25 nm thick.

As described above, because of the AlGaN/GaN heterobarrier at theinterface between the channel layer 16 and the barrier layer 18, a twodimensional electron gas is induced at the interface. An ohmic sourceelectrode 22 and an ohmic drain electrode 23 are formed on the surfaceof the barrier layer 18. The source 22 and drain 23 ohmic electrodes maybe Ti/Si/Ni, Ti/Al/Ni, Ti/Al/Ni/Au or any other suitable material thatforms an ohmic contact to n-type AlGaN. Appropriate ohmic contacts forAlGaN/GaN HEMT devices are described in S. T. Sheppard, W. L. Pribble,D. T. Emerson, Z. Ring, R. P. Smith, S. T. Allen and J. W. Palmour,“High Power Demonstration at 10 GHz with GaN/AlGaN HEMT HybridAmplifiers,” Presented at the 58th Device Research Conference, Denver,Colo. June 2000, and S. T. Sheppard, K. Doverspike, M. Leonard, W. L.Pribble, S. T. Allen and J. W. Palmour, “Improved 10-GHz Operation ofGaN/AlGaN HEMTs on Silicon Carbide,” Mat. Sci. Forum, Vols. 338-342(2000), pp. 1643-1646.

The distance between the source electrode 22 and the drain electrode 23is typically about 2-4 μm.

In some embodiments, a thin (20-40 Å), undoped GaN cap layer (not shown)may be formed on the surface of the barrier layer 18 between the source22 and the drain 23 ohmic electrodes. The design and effect of such caplayers is described in detail in U.S. application Ser. No. 09/904,333 toSmith for “ALUMINUM GALLIUM NITRIDE/GALLIUM NITRIDE HIGH ELECTRONMOBILITY TRANSISTORS HAVING A GATE CONTACT ON A GALLIUM NITRIDE BASEDCAP SEGMENT AND METHODS OF FABRICATING SAME” the disclosure of which ishereby incorporated by reference.

The gate electrode 24 is formed between the source 22 and the drain 23electrodes. In certain embodiments, the gate electrode 24 is formed ofplatinum, nickel or any other suitable metal that forms a non-ohmiccontact to n-type Al_(x)Ga_(1-x)N. The gate electrode 24 may be cappedwith an additional metal layer in a T-gate configuration, or a T-gatemay be formed in one process step. As is known in the art, T-gateconfigurations are particularly suitable for RF and microwave devices.

The barrier layer 18 may also be provided with multiple layers asdescribed in the above-referenced U.S. patent application Ser. No.10/102,272, to Smorchkova et al. Thus, embodiments of the presentinvention should not be construed as limiting the barrier layer to asingle layer but may include, for example, barrier layers havingcombinations of GaN, AlGaN and/or AlN layers. For example, a GaN, AlNstructure may be utilized to reduce or prevent alloy scattering.

As illustrated in FIG. 1A, a portion of the surface 25 of the bufferlayer 14, such as an AlN buffer layer, is exposed. A first plurality ofmetal fingers 26 forms an input IDT on the exposed surface 25, while asecond plurality of metal fingers 28 forms an output IDT. The input IDT26, the output IDT 28 and the AlN buffer layer 14 form a surfaceacoustic wave device 10B integrated on the same substrate as thetransistor 10A. The IDTs 26, 28 may be formed of aluminum or any othersuitable metal. Since the IDTs 26, 28 do not have to faun an ohmic orother electrical contact with the buffer layer 14, many different metalsmay be employed (including for example the same metals used for the gate24, the source 22 and drain 23 ohmic contacts and/or metalinterconnections to reduce the number of metallization steps required).

The thickness of the buffer layer 14 affects the SAW propagationcharacteristics of SAW structure 10B. In general, a surface acousticwave propagating in a device extends for a distance of about onewavelength into the structure. In some embodiments, the buffer layer isabout 250 nm or less. In that case, the SAW velocity of the propagatingwave may be dominated by the SiC substrate, which has a higher SAWvelocity than AlN. Also, the orientation of the SAW IDT fingers 26, 28may be chosen so as to optimize the performance of the device. Althoughit has been reported that SAW propagation is isotropic in the c-plane ofAlN and SiC, it is possible to fabricate devices using off-axis cut SiCsubstrates, which may make the propagation of surface acoustic wavesdependent on orientation. For example, SiC MESFETs are typicallymanufactured using 8° off-axis wafers.

The input IDT 26 or the output IDT 28 may be connected to the source 22,the drain 23 and/or the gate 24 of transistor 10A via an overlayer metal(not shown), or via wire bonding or any other suitable technique so thatelectrical signals can be communicated from transistor 10A to SAW device10B and vice versa.

To reduce and/or minimize undesired reflections and interference fromthe transistor 10A, the die may be cut at an angle different from theorientation of the IDT fingers 26, 28 as illustrated in FIG. 9. Inaddition, SAW reflectors and/or absorbers 19 may be formed on the deviceadjacent the input IDT 26 and/or the output IDT 28 to minimize undesiredinterference. The design of SAW reflectors and absorbers is well knownto those skilled in the design of SAW devices.

Precursor structures illustrating alternative methods of fabricating thedevice 10 are illustrated in FIGS. 1B and 1C. As illustrated in FIG. 1B,a precursor device 11 may be formed by depositing epitaxial layers on asubstrate 12 to form the buffer layer 14, the channel layer 16 and thebarrier layer 18. Prior to metallization, an etch mask 32 is formed onthe surface of the barrier layer 18. The etch mask 32 may comprisephotoresist, silicon dioxide, silicon nitride, or any other suitablemask that permits selective etching of the underlying nitride epitaxiallayers. The etch mask 32 is patterned using standard photolithographytechniques and partially removed. A typical photolithography process caninclude the steps of adding a layer of photoresist (typically a polymerresin sensitive to light) to a semiconductor structure, positioning amask over the photoresist, exposing the photoresist to a frequency oflight to which it responds (by undergoing a chemical change; usually itssolubility in a particular solvent), etching the photoresist to removethe exposed or unexposed pattern (depending upon the resist selected),and then carrying out the next desired step on the remaining pattern.

The barrier and channel layers 16, 18 are then etched away to reveal aportion of the MN buffer layer 14 on which the IDTs 26, 28 may beformed. The etch mask 32 is then removed and metallization steps areperformed to form the contacts 22, 23, 24 and the IDTs 26, 28.

The barrier 16 and the channel 18 layers may be etched using a dry etchprocess such as reactive ion etching (RIE). Suitable conditions for dryetching the mesa may include dry etching in an Ar environment using BCl₃etchant. For example, a typical process may include flowing Ar at 20-100seem and BCl₃ at 10-20 seem in an RIE reactor at a pressure of 5-50mTorr and an RF power at 50-300 W. Actual parameters will depend on thesystem used and may be determined by those skilled in the art. The etchshould be highly selective to etch GaN but not AlN.

An alternative method of fabricating device 10 may be understood withreference to the precursor structure 13 shown in FIG. 1C. The device 10may be fabricated by placing the substrate 12 into a growth reactor anddepositing the AlN buffer layer 14 on the substrate 12 as describedabove. After formation of the buffer layer 14, the substrate 12 isremoved from the growth reactor and a growth mask 34 (which may comprisesilicon dioxide, silicon nitride or another suitable material) is formedon the surface of the buffer layer. The mask layer 34 is patterned usingstandard photolithographic techniques as described above to reveal aportion of the surface of the buffer layer 14. After formation andpatterning of the mask layer 34, the substrate 12 is placed back into agrowth reactor for the regrowth of channel and the barrier layer 16 andthe channel layer 18 (and any other layers that may be present in thedevice). The contacts 22, 23, 24 and the IDTs 26, 28 may then be formedon the structure after removal of the mask layer 34.

As illustrated in FIGS. 2A and 2B, the device 20 includes a SAWstructure 20B that is isolated from a transistor structure 20A byforming a trench 36 between the structures that extends through bufferlayer 14. The trench 36 may extend into the substrate 12 depending onthe thickness of the buffer layer 14 and the wavelength of the device.As discussed above, a SAW extends for a distance of about one wavelengthinto the propagating medium. Thus, the trench 36 may extend for adistance of at least about one wavelength in order to provide physicalisolation of SAW device 20B.

As illustrated in FIG. 2A, the trench 36 may be formed before, during orafter formation of the transistor. For example, an etch mask 38 may beformed on the structure and patterned to reveal a portion of the bufferlayer 14 adjacent the transistor mesa. The exposed region is then etchedin the manner described above to provide device isolation. Afteretching, the etch mask is removed and metallization is performed asillustrated in FIG. 2B.

FIGS. 3A-3B illustrate other embodiments of the invention. As FIG. 3Aschematically illustrates, a device 30 includes a transistor structure30A and a SAW device structure 30B formed on a common substrate.However, in this embodiment, the IDTs 26, 28 of the SAW device 30B areformed on the surface of the same epitaxial layer as the transistorelectrodes, thus avoiding the need for regrowth or mesa etching.

In order to electrically isolate the SAW device 30B from the transistorstructure 30A and reduce loss in the SAW device itself, ions 43, such asnitrogen or phosphorus, are implanted into a region 42 of the device torender the region 42 sufficiently highly resistive as to be electricallyinactive.

As illustrated in FIG. 3B, after growth of the epitaxial layers of thetransistor structure 30A to form an epitaxial precursor structure 31, apatterned implant mask 44 (which may comprise photoresist) is formed onthe surface of the precursor structure 31 and nitrogen ions 43 areimplanted into the exposed surface 45 so as to implant the nitrogen ions43 into the region 42. Other ions can be used to increase theresistivity of the region via implantation, including hydrogen, helium,aluminum, nitrogen, magnesium, zinc and/or calcium.

The implantation can be carried out in a conventional fashion and atroom temperature. As presently best understood (and without beinglimited by a particular theory), the implanting ions create damagewithin the GaN to produce deep levels within the bandgap. These in turntrap free carriers in the GaN, thus rendering the material moreresistive than a GaN layer or region without such an implant. In a HEMTstructure, the implanted ions effectively neutralize the conductive 2DEGchannel at the interface between the barrier and channel layers.

In certain embodiments, nitrogen atoms are implanted into the exposedregion at an energy of 10-400 keV and a dosage of 10¹³-10¹⁴ ions persquare centimeter (cm⁻²). Such a dose may be sufficient to neutralizethe region 42 or otherwise make the region 42 sufficientlynon-conductive such that the transistor structure 30A is electricallyisolated from the SAW device 30B such that the electrical performance ofeither the transistor structure 30A or the SAW device 30B are notsubstantially impaired by the other device.

FIG. 4 illustrates a device 40 according to further embodiments of thepresent invention. As with the embodiments described in connection withFIGS. 3 and 3A, a SAW device 40B is formed on the same epitaxial surfaceas the transistor structure 40A. However, in addition to the implantneutralization of the layers forming the SAW device 40B, an isolationtrench 56 is formed between the SAW structure 40B and the transistorstructure 40A using the masking and etching techniques described abovein connection with FIGS. 2A and 2B. As discussed above, the depth of thetrench 56 may be equal to at least one SAW wavelength.

Further embodiments of the present invention are illustrated in FIG. 5.In the embodiments illustrated in FIG. 5, the barrier and the channellayers of the transistor structure 50A are etched down into the thickGaN layer 16 to remove the 2DEG region between the barrier and channellayers. The SAW IDTs are formed on the exposed surface of the GaN layer,which is semi-insulating as grown. An optional isolation trench 66 maybe etched between the transistor structure 50A and the SAW devicestructure 50B using the techniques described above.

In the embodiments illustrated in FIGS. 6A-6C, an AlN-based SAWstructure 60B is integrated on the same substrate as a SiC MESFETstructure 60A. Techniques for growing epitaxial layers of siliconcarbide are disclosed in U.S. Pat. Nos. 6,063,186; 6,297,522; 6,217,662;5,155,062; 4,946,547; 4,912,063; 4,912,064; and 5,011,549, thedisclosures of which are incorporated herein by reference. Techniquesfor growing epitaxial layers of silicon carbide are also disclosed inU.S. application Ser. No. 09/715,576 filed Nov. 17, 2000 for “SUSCEPTORDESIGNS FOR SILICON CARBIDE THIN FILMS”; U.S. application Ser. No.09/790,169 filed Feb. 21, 2001 for “SUSCEPTOR DESIGNS FOR SILICONCARBIDE THIN FILMS”; U.S. Publication No. 2001/0170491 published Nov.21, 2002 for “SEED CRYSTAL HOLDERS AND SEED CRYSTALS FOR FABRICATINGSILICON CARBIDE CRYSTALS AND METHODS OF FABRICATING SILICON CARBIDECRYSTALS”; U.S. Publication No. 2002/0090454 published Jul. 11, 2002 for“GAS-DRIVEN ROTATION APPARATUS AND METHOD FOR FORMING SILICON CARBIDELAYERS”; U.S. application Ser. No. 10/017,492 filed Oct. 30, 2001 for“INDUCTION HEATING DEVICES AND METHODS FOR CONTROLLABLY HEATING ANARTICLE”; and U.S. application Ser. No. 10/117,858 filed Apr. 8, 2002for “GAS-DRIVEN PLANETARY ROTATION APPARATUS AND METHODS FOR FORMINGSILICON CARBIDE LAYERS” the disclosures of which are incorporated hereinby reference.

The SiC epitaxial layers 72 may be grown using techniques described inthe foregoing patents and applications to form a precursor structure 61as illustrated in FIG. 6B. An etch mask 73 is formed on the surface ofthe SiC epitaxial layers 72 and patterned to reveal a portion of thesurface 74 of the SiC epitaxial layers 72. The precursor structure 61 isthen etched to reveal a portion of the semi-insulating SiC substrate.The remaining SiC epitaxial layers are then masked with a growth mask 77illustrated in FIG. 6C that extends just past the edge of the SiCepitaxial layers 72 by a predetermined distance that may depend on thethickness of the growth mask 77. The etch mask 73 may or may not beremoved prior to formation of the growth mask 77.

A layer of crystalline AlN 14 is regrown on the exposed substrate andthe mask 77 is removed. Because the regrown AlN layer 14 is separatedfrom the SiC epitaxial layer 72, the SAW structure 60B and the MESFETstructure 60A are isolated as grown. The metal contacts 22, 23, 24 areformed on the SiC epitaxial layers 72 and the SAW IDTs 26, 28 are thenformed on the AlN layer to complete the device.

As illustrated in FIG. 7, the foregoing techniques can be extended tointegrate more than one type of device on the same substrate as one ormore SAW devices. For example, the device 70 includes a transistorstructure 70A, a SAW device 70B and a photodetector structure 70Cintegrated on the same substrate 12. Such a device may, for example, beused as a monolithic element capable of receiving, amplifying, andfiltering an optical information signal. Likewise, the embodimentsillustrated in FIG. 8 include a transistor structure 80A, a SAW device80B, a photodetector structure 80C, and an emitter structure 80Dintegrated on the same substrate 12. Such devices may, for example, beused as a monolithic element capable of receiving, amplifying, andfiltering an optical information signal and transmitting an informationsignal. The design of GaN-based emitters and photodetectors is wellknown to those skilled in the art. Examples of GaN-based photodetectorsare illustrated in U.S. Pat. Nos. 6,495,852 and 6,265,727, which areincorporated herein by reference. Examples of GaN-based emitters areillustrated in U.S. Pat. Nos. 5,523,589 and 5,739,554, which areincorporated herein by reference.

Other circuit elements may be integrated onto the same substrate as theSAW device and the electronic device. For example, capacitors,inductors, resistors, delay lines and the like may be integrated intothe device as well.

Integration of different types of nitride devices on a common substrateposes challenges, because many types of devices, such as transistors,emitters, limiters, etc., have complex epitaxial structures which aretailored to the operation of a particular type of device. Embodiments ofthe present invention arise from a recognition that while certaindevices have different epitaxial structures, some aspects of thosestructures may be similar, and may be used in different types of devicesformed on a common substrate.

For example, FIG. 12A and FIG. 12B show exemplary epitaxial precursorstructures that may be used to form two different types of devices. Theepitaxial structure shown in FIG. 12A (Type I) may be used to fabricatea low-noise amplifier or a power amplifier, while the epitaxialstructure shown in FIG. 12B (Type II) may be used as a basis for alimiter or an RF switch transistor. In particular, the Type I epitaxialstructure of FIG. 12A includes a substrate 100 on which a buffer layer102 and channel layer 104 are formed. The substrate 100 may include asemi-insulating 6H or 4H silicon carbide substrate. The buffer layer 102may include an aluminum nitride layer, which may provide an appropriatecrystal structure transition between the silicon carbide substrate 100and the remainder of the device. In particular, the buffer layer 102 maybe formed by metal-organic chemical vapor deposition and may have athickness of about 100 Å to about 10,000 Å. In particular embodiments,the buffer layer 102 may have a thickness of about 1000 Å.

A channel layer 104, which in some embodiments is semi-insulatingAl_(x)Ga_(1-x)N (0≦x≦1), is formed on the buffer layer 102. In someembodiments, the channel layer 104 is undoped, but in other embodimentsmay be doped with various impurities in order to modify the electronconcentration in the sheet charge region or the behavior of theconduction band Ec and valence band Ev in the area below the sheetcharge region. In some embodiments, the channel layer 104 is about 1 to5 μm thick. In some embodiments, the channel layer 104 may include a 2μm thick GaN-based buffer layer with a thin (approximately 100 Å)high-quality channel region (not shown) formed thereon.

A barrier layer 106 is formed on the channel layer 104. In someembodiments, the barrier layer 106 may include AlN, thereby forming aheterojunction between the channel layer 104 and the barrier layer 106.A high bandgap layer 108 is on the barrier layer 106. The high bandgaplayer 108 may include Al_(x)Ga_(1-x)N (0≦x≦1) having an aluminumcomposition of between 20% and 40%. The high bandgap layer 108 may havea higher bandgap than the channel layer 104. The high bandgap layer 108may be doped with silicon at a doping concentration of about 2×10¹⁸cm⁻³. The high bandgap layer 108 may be between about 15 nm and 40 nm inthickness, and in some embodiments is about 25 nm thick.

As discussed above, because of the AlGaN/GaN heterobarrier at theinterface between the channel layer 104 and the barrier layer 106, a twodimensional electron gas may be induced at the interface.

A high purity silicon nitride layer 110 may be formed on the highbandgap layer 108. High purity silicon nitride may be formed asdescribed in commonly-assigned U.S. patent application Ser. No.11/286,805 filed Nov. 23, 2005 and entitled “Group III NitrideSemiconductor Devices with Silicon Nitride Layers and Methods ofManufacturing Such Devices” the disclosure of which is herebyincorporated by reference as if fully set forth herein.

The Type I epitaxial structure of FIG. 12A may be useful for forming aHEMT device for a low noise and/or high power amplifier. For example,the Type I epitaxial structure may provide appropriate peak current,transconductance, and/or breakdown voltage characteristics useful forhigh power amplifier output transistors. The thickness and/or thealuminum percentage of the high bandgap layer 108 may be adjusted toprovide appropriate characteristics for a low noise amplifier.

The epitaxial structure shown in FIG. 12B (Type II) is similar to theType I epitaxial structure shown in FIG. 12A, except that the Type IIepitaxial structure includes an additional epitaxial layer 112 betweenthe high bandgap layer 108 and the high purity silicon nitride layer110. In particular, the high bandgap layer 108, the barrier layer 106and the channel layer 104 may be the same as in the Type I epitaxialstructure. The additional epitaxial layer 112 may include a layer ofdoped GaN and/or graded AlGaN having a thickness of about 300 Å to about1000 Å. For an RF switch device and/or a limiter device, the additionalepitaxial layer 112 may increase the distance between the top surface ofthe device and the charge in a 2DEG channel induced at the interfacebetween the channel layer 104 and the barrier layer 106. This may reducethe gate-to-source capacitance of the device, while maintaining a lowchannel resistance.

A device having a Type I epitaxial structure and a device having a TypeII epitaxial structure may be integrated on a common substrate asillustrated in FIG. 13A. As shown therein, a Type II epitaxial structuremay be formed using, for example, conventional MOCVD growth techniques.Source/drain ohmic contacts 122, 124, 128 may be recessed through thehigh purity silicon nitride layer 110 and the additional epitaxial layer112. While not illustrated in FIG. 13A, the source/drain ohmic contacts122, 124, 128 may be fully and/or partially recessed into/through thehigh bandgap layer 108 and/or the barrier layer 106.

A first transistor Q1 is defined by a gate 120 positioned betweenadjacent source/drain contacts 122, 124. The first transistor Q1 may be,for example, a high power or low-noise transistor. Thus, in the firsttransistor Q1, the gate contact 120 is recessed through both the highpurity silicon nitride layer 110 and the additional epitaxial layer 112,as shown in FIG. 13A.

Accordingly, for the first transistor Q1, the gate etch is used toremove a portion, or all, of the additional epitaxial layer 112underneath the high purity silicon nitride layer 110. A low channel oraccess resistance may be maintained in the non-gated regions of thefirst transistor Q1 due to the presence of the additional epitaxiallayer 112, which, as explained above, may be a relatively thick GaN capand/or graded/doped AlGaN as described in Journal of ElectronicMaterials, Vol. 33, No. 5, 2004, or IEEE Electron Device Letters, Vol.25, No. 1, January 2004 or Journal Of Applied Physics Volume 94, Number8, 15 Oct. 2003.

A second transistor Q2 is defined by a second gate contact 126positioned between source/drain contacts 124, 128. The second transistorQ2 may be configured, for example, as a limiter and/or an RF switch. Thesecond gate contact 126 may be recessed only through the high puritysilicon nitride layer, and is not recessed through, or is only partiallyrecessed through, the additional epitaxial layer 112. The presence ofthe additional epitaxial layer 112 may reduce the gate capacitance ofthe second transistor Q2. Thus, two transistors Q1 and Q2, havingdifferent epitaxial structures and/or different operationalcharacteristics, may be provided on the same substrate by forming asingle epitaxial structure and etching the structure appropriately.

A schematic diagram of a possible circuit formed by the first and secondtransistors Q1 and Q2 is illustrated in FIG. 13B. As shown therein, thefirst and second transistors may share a common source/drain contact124.

Further methods and structures according to embodiments of the inventionare illustrated in FIGS. 14A and 14B. As shown therein, a Type Iepitaxial structure similar to the structure illustrated in FIG. 12A isprovided. In particular, the structure shown in FIG. 14A includes abuffer layer 102 on a substrate 100. A channel layer 104, a barrierlayer 106, a high bandgap layer 108 and a high purity silicon nitridelayer 110 are formed on the buffer layer 102. A photoresist mask 210 isformed on a portion of the high purity silicon nitride layer 110, and aportion of the high purity silicon nitride layer exposed by the mask 210(i.e. not covered by the mask 210) may be etched away using, forexample, conventional dry etching techniques. The exposed portions ofthe high purity silicon nitride layer 110 may be etched away, forexample, to reveal the high bandgap layer 108 beneath the high puritysilicon nitride layer 110.

Following the selective etching of the high purity silicon nitridelayer, the photoresist mask 210 may be removed, and a epitaxial layer125 may be epitaxially regrown on the exposed high bandgap layer 108.The epitaxial layer 125 may be similar to the epitaxial layer 112 shownin FIG. 13A. For example, the epitaxial layer 125 may include doped GaNand/or graded AlGaN, and may have a thickness of about 30 Å to about1000 Å.

The high purity silicon nitride layer 110 may act as a growth mask forthe growth of the epitaxial layer 125. That is, when the epitaxial layer125 is regrown, for example using MOCVD, crystalline material may notnucleate and/or grow on the high purity silicon nitride layer 110, asthe high purity silicon nitride layer 110 may not provide a suitabletemplate for epitaxial growth.

In some embodiments, however, it may be desirable to provide a separategrowth mask on the high purity silicon nitride layer 110 when theepitaxial layer 125 is regrown in order to reduce and/or prevent straynucleation on the silicon nitride layer 110. The photoresist mask 210used as an etch mask when the high purity silicon nitride layer 110 isetched may not be useful as a growth mask, because the polymer-basedmaterial used in photoresist may not withstand the high temperaturesrequired for epitaxial growth of nitride materials. Thus, a separategrowth mask may be provided on the high purity silicon nitride layer110. For example, referring to FIG. 14B, a growth mask 160 may be formedon the high purity silicon nitride layer 110. The growth mask mayinclude, for example, silicon dioxide and/or aluminum nitride. Thegrowth mask may be formed on the high purity silicon nitride layer 110prior to etching the high purity silicon nitride layer 110, and may bepatterned using conventional lithographic techniques. For a silicondioxide growth mask, the silicon dioxide should have a thickness greaterthan 1000 Å.

In some embodiments, the same etch mask 210 may be used to pattern boththe etch mask 160 and the high purity silicon nitride layer 110. Inother embodiments, the etch mask 210 may be used to pattern the growthmask 160, and the growth mask 160 may be used as an etch mask foretching the high purity silicon nitride layer 110, as well as forpreventing and/or reducing unwanted nucleation on the high puritysilicon nitride layer 110 during epitaxial regrowth of the epitaxiallayer 125.

As shown in FIG. 14B, during epitaxial regrowth of the epitaxial layer125, some random crystallite and/or polycrystalline material 163 maynucleate and/or grow on the growth mask 160. The unwanted material 163may be removed by removing the growth mask 160 from the high puritysilicon nitride layer 110, which may be done, for example, by performinga buffered oxide etch (BOE) if the growth mask 160 is silicon dioxide. ABOE may etch silicon dioxide but may not etch high purity siliconnitride 110.

Following regrowth of the epitaxial layer 125, a plurality of devicesmay be defined in the epitaxial structure by forming one or moreelectrical contacts on the structure. For example, as shown in FIG. 15,a first transistor Q3, which may be a low noise amplifier and/or a highpower amplifier, may be defined by forming source/drain contacts 132,134 on the high bandgap layer 108. The source/drain contacts 132, 134may be partially and/or fully recessed through the high bandgap layer108. A gate contact 130 for the first transistor Q3 is recessed throughthe high purity silicon nitride layer 110. In some embodiments, thetransistor Q3 may have an insulating gate structure (e.g. ametal-insulator-semiconductor heterojunction field effect transistor, orMISHFET) as shown in any of U.S. Pre-grant Publication No. 2003/0020092entitled “Insulating Gate AlGaN/GaN HEMT”, U.S. Pre-grant PublicationNo. 2005/0170574 entitled “Nitride-based Transistors with a ProtectiveLayer and Low-damage Recess and Method of Fabrication Thereof,” U.S.patent application Ser. No. 11/185,398, filed Jul. 20, 2005 and entitled“Nitride-Based Transistors and Fabrication Methods With an Etch StopLayer” and/or U.S. patent application Ser. No. 11/187,171, filed Jul.21, 2005 and entitled “Switch Mode Power Amplifier using MIS-HEMT withField Plate Extension,” the disclosures of which are incorporated hereinby reference as if fully set forth herein.

A second transistor Q4 may be formed in the region of the structure inwhich the epitaxial layer 125 was regrown. In particular, the secondtransistor Q4 may be defined by a gate contact 140 that is recessedthrough a silicon nitride layer 170 that may be formed on the epitaxiallayer 125. Source and drain contacts 142, 144 complete the secondtransistor Q4.

It will be appreciated that the second transistor Q4 may be configuredas a two terminal device, such as a limiter diode, by electricallyconnecting the gate contact 140 and the drain contact 144. When soconfigured, the gate and drain contacts 140, 144 together function as adevice anode, while the source contact 142 may function as a devicecathode. A limiter diode may be used, for example, as an input to anamplifier transistor, such as the first transistor Q3.

It will be further appreciated that the epitaxial layers of the firsttransistor Q3 may serve as the substrate on which the epitaxial layersof the second transistor Q4 are formed. Accordingly, some embodiments ofthe invention provide a first nitride based electronic device includinga first nitride based epitaxial structure, and a second nitride basedelectronic device having a second nitride based epitaxial structureformed on the first nitride based epitaxial structure. In the case ofthe structure shown in FIG. 15, the buffer layer 102, channel layer 104,barrier layer 106 and high bandgap layer 108 form a first epitaxialstructure for a first transistor Q3, while the epitaxial layer 125 formsa second epitaxial structure, formed on the first epitaxial structure,for a second transistor Q4.

Referring to FIG. 16, In some embodiments in which a limiter diode isformed, it may be desirable to provide a first epitaxially regrown layer127 of n+ GaN on the high bandgap layer 108 and a second epitaxiallyregrown layer 129 of n− GaN on the first regrown layer 127. The firstregrown layer 127 may have a thickness of about 0.5 μm and a dopingconcentration of about 2×10¹⁸ cm⁻³ or more, which provides a surfacecharge density of at least about 1×10¹⁴ cm⁻². The second regrown layer129 may have a thickness of at least 1000 Å, and may be doped lightlyenough that it is fully depleted at zero bias. In particular, the secondregrown layer 129 may have a doping concentration of less than about1×10¹⁶ cm⁻³.

Embodiments of the present invention may provide both enhancement anddepletion mode nitride-based transistor devices on a common substrate.For example, a depletion mode device may be formed by providing anepitaxial structure, such as the Type I epitaxial structure shown inFIG. 12A, and forming ohmic contacts thereto as described in connectionwith FIG. 14C. An enhancement mode device may be formed using the sameepitaxial structure, but by recessing the gate contact into theunderlying high bandgap layer 108.

Some embodiments of the invention provide for the integration of bothsilicon carbide devices and nitride devices on a common substrate. Forexample, as illustrated in FIG. 6A, a silicon carbide MESFET device anda nitride based SAW device may be integrated on a common substrate. Asdiscussed above in connection with FIG. 6A, it may be possible to forman epitaxial layer 72 of silicon carbide on a substrate on which a layer14 of aluminum nitride is also formed.

However, it may be difficult to form both silicon carbide epitaxiallayers and gallium nitride-based epitaxial layers on a common substrate,since gallium nitride based epitaxial layers are conventionally grown onon-axis substrates, while silicon carbide epitaxial layers areconventionally grown on off-axis substrates, although it is possible togrow relatively thin (<100 nm) epitaxial layers of silicon carbide on anon-axis substrate.

According to some embodiments of the invention, a silicon carbide MESFETmay be formed on an on-axis substrate by implanting source/drain regionsas well as a channel region in an on-axis semi-insulating siliconcarbide substrate, as illustrated in FIGS. 17A and 17B. As showntherein, one or more implant regions may be formed in an on-axis,semi-insulating 4H—SiC substrate by means of ion implantation. Forexample, as shown in FIG. 17A, an n-type channel region 216 may beformed in the substrate 200 by selective implantation of nitrogen and/orphosphorus ions. Furthermore, n+ source/drain regions 212, 214 may beformed in the substrate 200 by ion implantation. Multiple implantationsteps with different implant energies/doses may be performed in order toprovide a desired doping profile. Implantation of dopants into bulksilicon carbide layers is known in the art. After implantation, thedopants may be activated by annealing the implanted structure at atemperature of about 1400° C. to about 1700° C. for about 5 minutes toabout 30 minutes. In particular, it may be desirable to activate theimplanted dopants prior to epitaxial growth of nitride layers on thesubstrate 200, as the temperatures required to activate dopantsimplanted in silicon carbide may be detrimental to nitride-basedepitaxial layers.

As further illustrated in FIG. 17A, The implanted regions of thesubstrate 200 may then be masked with a growth mask 205. The growth mask205 may include, for example, a SiN and/or SiO₂ pattern formed on thesurface of the substrate 200. An epitaxial structure 210 may be grown onthe exposed portions of the substrate 200 using conventional epitaxialgrowth techniques. For example, the epitaxial structure 210 may includea buffer layer, a channel layer, a barrier layer, and a high bandgaplayer, which may have structures similar to the corresponding layersshown, for example, in FIG. 12A, in order to provide an epitaxialprecursor for a HEMT device. However, it will be appreciated that otherepitaxial structures could be formed, depending on thetype/characteristics of nitride device desired.

Referring to FIG. 17B, the growth mask 205 may be removed, and apassivation layer 228, such as silicon dioxide and/or silicon nitride,may be formed on exposed portions of the substrate 200 and the epitaxialstructure 210. Alternatively, the growth mask may be left on thesubstrate as a passivation layer, and a second passivation layer may beformed on the nitride epitaxial layers. Accordingly, while a singlepassivation layer 228 is illustrated in FIG. 17B, the passivation layeron the nitride epitaxial structure 210 may be different from thepassivation layer on the silicon carbide substrate 200. Furthermore, thepassivation layer 228 may include multiple passivation layers of thesame and/or different types of materials stacked on one another.

A first gate contact 230 and corresponding source/drain contacts 232,234 may be formed on the epitaxial structure 210 to define a HEMTtransistor device Q5, and a second gate contact 240 may be formed on theimplanted channel region 216 along with corresponding source/draincontacts 242, 244, to define a silicon carbide MESFET device Q6 in thesubstrate 200. The nitride device Q5 and the silicon carbide device Q6may be interconnected on the common substrate 200 by one or more levelsof interconnect metallization. For example, a dielectric layer 260 maybe formed on the nitride device Q5 and the silicon carbide device Q6.Metal contact plugs 261, 262 may be formed through the dielectric layer260 to contact electrical contacts of the nitride device Q5 and thesilicon carbide device Q6, respectively. The contact plugs 261, 262 maybe electrically connected, for example, by an overlayer metal 266 formedon the dielectric layer 260. Other connections between the nitridedevice Q5 and the silicon carbide device Q6 are possible.

Accordingly, in some embodiments of the invention, a silicon carbidetransistor device may be formed on the same substrate as a nitride-basedtransistor device to form a monolithically integrated device structure.Monolithically integrated nitride devices, and monolithically integratedsilicon carbide/nitride devices, may have particular applications inmonolithic microwave integrated circuits, in which it may be desirableto carefully control the orientation, placement and/or interconnectionof devices, due to the high frequency and/or high power of signalsprocessed by such devices. For example, when devices are notmonolithically integrated, it may be necessary to interconnect suchdevices using electrical traces and/or wires, which may add unwantedinductance, capacitance and/or resistance to the circuit. Suchinterconnections may also increase the difficulty of circuit design,since the circuit designer must take into account additional signaldelays and/or signal dispersion associated with chip-to-chipinterconnections.

It will be appreciated that the device structures illustrated herein maybe representative unit cell structures that are replicated in largerdevices, which may have multiple gate fingers on multiple activeregions. Furthermore, while some embodiments of the invention employsemi-insulating silicon carbide substrates, other types and/orconductivities of substrates may be utilized.

While embodiments of the present invention have been described withreference to isolation of the device(s) from other devices on a commonsubstrate through the use of etching and/or selective growth processes,other techniques of trench formation such as sawing, laser ablation orother techniques known to those of skill in the art may be utilized toprovide such isolation trenches. Device isolation may also be providedby isolation implants or other means.

Embodiments of the invention have been set forth in the drawings andspecification, and although specific terms have been employed, they areused in a generic and descriptive sense only and not for purposes oflimitation, the scope of the invention being defined in the claims.

That which is claimed is:
 1. A monolithic electronic device, comprising:a bulk, semi-insulating silicon carbide substrate; source and drainregions in a surface of the bulk, semi-insulating silicon carbidesubstrate; a channel region in the bulk, semi-insulating silicon carbidesubstrate between the source and drain regions, and wherein the sourceand drain regions comprise implanted source and drain regions, andwherein the channel region comprises an implanted channel region; anitride epitaxial structure on the surface of the bulk, semi-insulatingsilicon carbide substrate; and a first passivation layer on theimplanted channel region and the implanted source and drain regions ofthe bulk, semi-insulating silicon carbide substrate; a first gatecontact on the bulk, semi-insulating silicon carbide substrate betweenthe implanted source and drain regions, the first gate contact extendingthrough the first passivation layer.
 2. The monolithic electronic deviceof claim 1, further comprising: a second gate contact on the nitrideepitaxial structure.
 3. The monolithic electronic device of claim 2,wherein the first passivation layer extends onto the nitride epitaxialstructure, and the second gate contact extends through the firstpassivation layer.
 4. The monolithic electronic device of claim 2,further comprising: a second passivation layer on the nitride epitaxialstructure, the second passivation layer comprising a material differentfrom the first passivation layer; and wherein the second gate contactextends through the second passivation layer.
 5. The monolithicelectronic device of claim 2, wherein the first gate contact is acontrol contact for a silicon carbide-based transistor device and thesecond gate contact is a control contact for a nitride-based transistordevice, and wherein the electronic device further comprises aninterconnect metallization connecting the silicon carbide-basedtransistor device and the nitride-based transistor device.
 6. Amonolithic electronic device, comprising: a bulk, semi-insulatingsilicon carbide substrate; first and second implanted regions in asurface of the bulk, semi-insulating silicon carbide substrate, thefirst implanted region having a first conductivity type and the secondimplanted region having a second conductivity type opposite the firstconductivity type, the first and second implanted regions having ametallurgical junction therebetween forming a p-n junction; and anitride epitaxial structure on the surface of the bulk, semi-insulatingsilicon carbide substrate.
 7. The monolithic electronic device of claim6, further comprising: a first passivation layer on the implanted regionof the silicon carbide substrate; a first contact on the first implantedregion of the silicon carbide substrate and a second contact on thesecond implanted region of the silicon carbide substrate, the first gatecontact extending through the passivation layer; and a gate contact onthe nitride epitaxial structure.